1. Field of the Invention
The present invention relates generally to radio frequency (RF) devices, and particularly to stripline transmission device packages.
2. Technical Background
Stripline devices are used in RF and microwave circuit applications. A stripline transmission line is implemented by sandwiching a planar conductor between a pair of ground planes. A dielectric material is interposed between the planar conductor and each of the ground planes. Of course, transmission lines are used to interconnect circuit elements and implement impedance transformation networks. Stripline structures may also be used to realize directional couplers, baluns, power dividers and other such devices commonly used in RF and microwave circuits.
A coupler is a four-port device that includes a primary transmission line disposed in close proximity with a secondary transmission line. Power is directed into the device by way of an input port connected to the primary transmission line. The power propagates along the primary transmission line and the electromagnetic waves are coupled onto the secondary line. The coupler structure, therefore, would be implemented in a stripline configuration by disposing the primary conductor and the secondary conductor on opposite sides of a very thin dielectric material. The coupled structure is subsequently disposed between the two ground planes in the manner previously described. Of course, baluns, power combiners, power splitters and the like may be implemented using coupler structures. Those of ordinary skill in the art will understand that a coupler is synonymous with the term “power combiner” because it couples or combines two input signals and provides a single combined output. One the other hand, those of ordinary skill in the art will also understand that if the coupler is used such that a single signal is directed into an input, the coupler will split the signal into two output signals. Those of ordinary skill in the art will understand that the coupler is employed, therefore, as a power splitter. A coupler may also be a hybrid coupler, i.e., the two signals that are output are 90° out of phase with respect to one another. Those of ordinary skill in the art will understand that two couplers may be used in tandem to form a balun structure, See, for example, FIG. 9 which shows a balun constructed using a first and a second backward wave symmetrical couplers.
The stripline devices discussed above may be implemented in surface mounted packages. Surface mounted devices are advantageous because they are dimensionally small (i.e., less than 1.0 inch) and typically have a low profile (less than 0.15 inches). Of course, the dimensions will vary as a function of power and frequency. Accordingly, these devices are ideally suited for relatively small printed circuit boards used in wireless infrastructure applications, such as power chips, etc.
In one approach, the manufacturing process produces a rectangular panel having a two-dimensional array of devices disposed thereon. The panel is implemented by sandwiching several layers of softboard dielectric materials. The stripline structure itself is formed using a very thin softboard layer (less than 10 mils) that includes copper foil disposed on either side. The array of stripline structures are typically formed by using standard photolithography techniques, i.e., the array of stripline devices are imaged onto the copper surfaces and subsequently etched, removing any excess copper material. This process is quite accurate and allows the placement of coupled transmission lines on either side of the thin panel within very high tolerances. A relatively thicker softboard dielectric panel (e.g., ˜less than 60 mils) is disposed on either side of the stripline structure. The thicker panels have a dielectric surface positioned next to the stripline structure and an exterior copper foil surface that functions as the ground plane. The softboard sandwich is laminated by applying heat and pressure. The laminated panel is easily cut and divided into its constituent individual components.
One of the benefits of the approach briefly described above is its extreme accuracy. The process is well understood and produces very smooth and well defined lines. Device performance parameters such as amplitude balance, phase balance, insertion loss, etc. are quite predictable. The process is very efficient, very large panels may be produced using very high levels of automation. Thus, the method is very conducive to low cost, high volume manufacturing. However, one drawback associated with this technique relates to the poor thermal conductivity of softboard materials. As those of ordinary skill in the art will appreciate, thermal conductivity is proportional to the amount of power that the individual component is able to dissipate.
What is needed, therefore, is a method for making stripline devices that is characterized by all of the above advantages associated with softboard devices, while at the same time, being characterized by high thermal characteristics and high power handling capabilities.